Synchronization of fluidic actuators

ABSTRACT

A fluidic system is disclosed. The system comprises a plurality of fluidic oscillatory actuators, and at least one synchronization conduit connecting two or more of the actuators such as to effect synchronization between oscillations in the two or more connected actuators.

RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.14/354,579 filed on Apr. 27, 2014, which is a National Phase of PCTPatent Application No. PCT/IB2012/055887 having International FilingDate of Oct. 25, 2012, which claims the benefit of priority of U.S.Provisional Patent Application Nos. 61/551,994 filed on Oct. 27, 2011,and 61/697,402 filed on Sep. 6, 2012. The contents of the aboveapplication are all incorporated by reference as if fully set forthherein in their entirety.

FIELD AND BACKGROUND OF THE INVENTION

The present invention, in some embodiments thereof, relates to fluidflow and, more particularly, but not exclusively, to a method and systemfor synchronizing fluidic actuators.

Fluid flow separation can occur when a compressible or incompressiblefluid flows over a surface, in particular a convex curved surface, suchas an interior surface of a fluid conduit or an exterior surface of abody immersed in a fluid. Flow separation can occur under laminar orturbulent flow conditions, depending upon the boundary layer fluid flowcharacteristics and the geometry of the surface. It is often desirableto inhibit flow separation in order to reduce form drag or in order toincrease aerodynamic lift. In general, the farther along a curvedsurface that a fluid travels before separation, the lower the resultingform drag and the higher aerodynamic lift.

Flow control technology relates generally to the capability to alterflow properties relative to their natural tendencies by introduction ofa constant or periodic excitation. Flow control systems can becategorized into Passive Flow Control (PFC) systems or Active FlowControl (AFC) systems. PFC systems provide substantially constant flowcontrol, whereas AFC systems allow flow control to be selectivelyapplied to a surface in contact with the fluid.

AFC systems are typically utilized to inhibit or delay separation of thefluid flow over the surface. Known AFC systems typically includeactuators or actuator arrays for introducing or removing perturbationsto the flow and altering the flow characteristics near the surface.

U.S. Pat. No. 7,055,541 discloses a suction and periodic excitation flowcontrol mechanism. The mechanism includes: a jet of fluid at acontrolled input pressure which is directed by control pressure gradientbetween two opposite ports at the sides of the jet. The mechanism alsocomprises a suction slot for allowing additional fluid to join the flowand create an amplified flow. An oscillating deflection device directsthe amplified flow in two or more exit directions.

U.S. Published Application No. 20100193035 discloses an automaticmechanism to produce a fluid jet with an oscillating exit direction. Themechanism includes a conduit which conveys a flow of fluid, a feedbackcontrol tube terminating in two control ports connected to one anotherby the feedback tube, and means for varying the effective diameter ofthe feedback control tube.

U.S. Published Application No. 20100194142 discloses a technique forreducing aerodynamic drag on a body moving through a fluid. A flowgenerator is mounted adjacent a blunt rear edge of the body. The flowgenerator generates a flow which controls an external flow at an edge ofthe body, wherein the flow of air oscillates in a direction parallel tothe blunt edge.

SUMMARY OF THE INVENTION

According to an aspect of some embodiments of the present inventionthere is provided a fluidic system. The system comprises: a plurality offluidic oscillatory actuators, and at least one synchronization conduitconnecting two or more of the actuators such as to effectsynchronization between oscillations in the two or more connectedactuators.

According to some embodiments of the invention the synchronizationconduits are effective to provide the synchronization devoid of anymoving non-fluidic parts.

According to some embodiments of the invention at least one of theactuator is a suction and oscillatory blowing actuator (SaOB).

According to some embodiments of the invention the synchronizationconduits are selected to control phase lag among the actuators.

According to some embodiments of the invention the system comprises atleast one feedback conduit.

According to some embodiments of the invention the synchronizationconduit(s) is constituted to effect opposite oscillations among at leastone pair of oscillatory actuators.

According to some embodiments of the invention a frequency of theoscillatory actuators is X times an expected vortex shedding frequencyof fluid at the vicinity of the system, wherein X is from about 1.5 toabout 3.5. According to some embodiments of the invention the X equals2.

According to some embodiments of the invention a distance betweenadjacent fluidic oscillatory actuators is about X v St/f, wherein v isan expected velocity of fluid at the vicinity of the system, f is anexpected vortex shedding frequency of fluid at the vicinity of thesystem, St is an expected Strouhal number characterizing fluidicoscillations, and X is from about 0.5 to about 4.

According to some embodiments of the invention each oscillatory actuatorcomprises two opposing control ports, wherein each control port of eachoscillatory actuator is respectively connected to at least two controlports of at least two another oscillatory actuators.

According to some embodiments of the invention each oscillatory actuatorcomprises two opposing control ports, wherein each control port of eachoscillatory actuator is connected to only one control port of anotheroscillatory actuator.

According to an aspect of some embodiments of the present inventionthere is provided an active separation control system, attachable to therear end of a vehicle and comprising the system as delineatedhereinabove and/or optionally as further detailed hereinbelow.

According to some embodiments of the invention the system comprises aflexible member, wherein the fluidic oscillatory actuators and thesynchronization conduit(s) are mounted on the a flexible member, andwherein the system is mountable on a rotatable door of the vehicle suchthat the flexible member assumes a different shape when the door isclosed than when the door is open.

According to some embodiments of the invention the vehicle is a trucktrailer.

According to some embodiments of the invention the vehicle is anaeronautical vehicle.

According to an aspect of some embodiments of the present inventionthere is provided a vehicle, comprising the system as delineatedhereinabove and/or optionally as further detailed hereinbelow.

According to an aspect of some embodiments of the present inventionthere is provided an active separation control system, attachable to anobject selected from the group consisting of an airfoil, a wing or afuselage, and comprising the system as delineated hereinabove and/oroptionally as further detailed hereinbelow.

According to an aspect of some embodiments of the present inventionthere is provided an active separation control system. The systemcomprises a fluidic oscillatory actuator having an ejector member and anoscillator member both mounted on a flexible member. The fluidicoscillatory actuator is mountable on a rotatable door of a vehicle suchthat the flexible member assumes a different shape when the door isclosed than when the door is open.

According to an aspect of some embodiments of the present inventionthere is provided a method of synchronizing fluidic oscillatoryactuators. The method comprises establishing fluid flow within at leastone synchronization conduit connecting at least two actuators such as toeffect synchronization between oscillations in the at least twoactuators.

According to some embodiments of the invention the method wherein eachactuator is connected by a respective synchronization conduit to allother actuators in the array.

According to some embodiments of the invention the method comprisescontrolling a phase lag among the actuators.

According to some embodiments of the invention the method comprisesestablishing a feedback flow between opposite ports of the sameactuator.

According to some embodiments of the invention the method comprisesgenerating opposite oscillations among at least one pair of oscillatoryactuators.

According to some embodiments of the invention each oscillatory actuatorcomprises a set of suction openings, and the method comprises closing atleast one of the suction openings so as to effect the magnitude of aspatial wave generated in the actuators.

According to some embodiments of the invention each oscillatory actuatorcomprises a plurality of pulsed blowing slots, wherein the closing ofthe at least one of the suction openings is so as to effect a locationof magnitude of the spatial wave relative to the pulsed blowing slots.

According to some embodiments of the invention the method is executedfor actively controlling the flow at a fluid boundary layer.

According to some embodiments of the invention the method is executedfor actively controlling the flow over a bluff body.

According to some embodiments of the invention the method is executedfor actively controlling wake flow.

According to some embodiments of the invention the method is executedfor actively controlling lift.

According to some embodiments of the invention the method is executedfor actively controlling moment acting on a fuselage, a rocket or anaircraft.

According to some embodiments of the invention the method is executedfor actively reducing drag.

Unless otherwise defined, all technical and/or scientific terms usedherein have the same meaning as commonly understood by one of ordinaryskill in the art to which the invention pertains. Although methods andmaterials similar or equivalent to those described herein can be used inthe practice or testing of embodiments of the invention, exemplarymethods and/or materials are described below. In case of conflict, thepatent specification, including definitions, will control. In addition,the materials, methods, and examples are illustrative only and are notintended to be necessarily limiting.

Implementation of the method and/or system of embodiments of theinvention can involve performing or completing selected tasks manually,automatically, or a combination thereof. Moreover, according to actualinstrumentation and equipment of embodiments of the method and/or systemof the invention, several selected tasks could be implemented byhardware, by software or by firmware or by a combination thereof usingan operating system.

For example, hardware for performing selected tasks according toembodiments of the invention could be implemented as a chip or acircuit. As software, selected tasks according to embodiments of theinvention could be implemented as a plurality of software instructionsbeing executed by a computer using any suitable operating system. In anexemplary embodiment of the invention, one or more tasks according toexemplary embodiments of method and/or system as described herein areperformed by a data processor, such as a computing platform forexecuting a plurality of instructions. Optionally, the data processorincludes a volatile memory for storing instructions and/or data and/or anon-volatile storage, for example, a magnetic hard-disk and/or removablemedia, for storing instructions and/or data. Optionally, a networkconnection is provided as well. A display and/or a user input devicesuch as a keyboard or mouse are optionally provided as well.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

Some embodiments of the invention are herein described, by way ofexample only, with reference to the accompanying drawings. With specificreference now to the drawings in detail, it is stressed that theparticulars shown are by way of example and for purposes of illustrativediscussion of embodiments of the invention. In this regard, thedescription taken with the drawings makes apparent to those skilled inthe art how embodiments of the invention may be practiced.

In the drawings:

FIGS. 1A-1F are schematic illustrations of an oscillatory blowingactuator, according to some embodiments of the present invention;

FIG. 2 is a schematic illustration of a fluidic system, according tosome embodiments of the present invention;

FIG. 3 shows form drag coefficient as a function of an inlet pressure ofan the actuators as measured during experiment performed according tosome embodiments of the present invention;

FIG. 4 shows results of increasing the phase lag between two valves byreducing the cross section of the synchronization tube as obtainedduring experiment performed according to some embodiments of the presentinvention;

FIG. 5 is a schematic representation in a lateral cross sectional viewof a system according to an exemplary embodiment of the inventioninstalled on a blunt-body;

FIGS. 6A and 6B are schematic illustrations showing a representativeexample of an active separation control system having a flexible member;

FIG. 7 is a schematic illustration of a two-dimensional truck model usedin experiment performed according to some embodiments of the presentinvention;

FIGS. 8A-8C are schematic illustrations of an actuator used inexperiment performed according to some embodiments of the presentinvention;

FIGS. 9A-9B show maximum and minimum flow velocity out of a singleactuator with an exit assembly as obtained in experiment performedaccording to some embodiments of the present invention;

FIGS. 10A and 10B show the velocity (FIG. 10A) and frequency (FIG. 10B)of oscillation of an actuator array installed in the circular cylinderand tested on a bench-top set-up during an experiment performedaccording to some embodiments of the present invention;

FIG. 11A is an illustration of a cross section of a 15-valves actuatorarray as installed inside the cylinder, and used an experiment performedaccording to some embodiments of the present invention;

FIG. 11B is a drawing of an image of an experimental set-up including acylinder in a Meadow-Knapp wind tunnel;

FIGS. 12A-12B show cylinder drag coefficient as a function of suctionposition in free laminar flow conditions (no truck model), as obtainedin an experiment performed according to some embodiments of the presentinvention;

FIGS. 13A-13D show a 2D truck model with an actuator installed on itsupper aft corner, used in an experiment according to some embodiments ofthe present invention;

FIGS. 14A-14B show the effect of the actuation on the drag and requiredpower, as measured in an experiment performed according to someembodiments of the present invention;

FIGS. 15A-15B show the effect of actuation on the baseline andcontrolled drag of the 2D truck model, as measured in an experimentperformed according to some embodiments of the present invention;

FIG. 16A is a schematic illustration of a 2D truck model with twocontrol cylinders, used in an experiment performed according to someembodiments of the present invention;

FIG. 16B is a drawing of an image showing a close-up rear-view of thelower aft-corner control cylinder (see “view I” in FIG. 16A);

FIG. 16C shows drag reduction as obtained in the experiment performedusing the model shown in FIGS. 16A and 16B;

FIG. 17 shows the relative drag reduction dependency on the reducedfrequency, as obtained in an experiment performed according to someembodiments of the present invention;

FIG. 18 shows the effect of the spatial waveform on the dragcoefficient, according to some embodiments of the present invention;

FIGS. 19A-19C illustrate several configurations for the opening ofsuction holes, according to some embodiments of the present invention;

FIG. 20A is a schematic illustration of a configuration in which asynchronization of an array of actuators is performed using a pluralityof parallel synchronization conduits connecting each side control ports,according to some embodiments of the present invention; and

FIG. 20B is a schematic illustration of a configuration in which asingle conduit connects ports of the same side (port A to port A andport B to port B), according to some embodiments of the presentinvention.

DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION

The present invention, in some embodiments thereof, relates to fluidflow and, more particularly, but not exclusively, to a method and systemfor synchronizing fluidic actuators.

Before explaining at least one embodiment of the invention in detail, itis to be understood that the invention is not necessarily limited in itsapplication to the details of construction and the arrangement of thecomponents and/or methods set forth in the following description and/orillustrated in the drawings and/or the Examples. The invention iscapable of other embodiments or of being practiced or carried out invarious ways.

Some embodiments of the present invention comprise a method andapparatus suitable for synchronizing an array of unsteady fluidicactuators, such as, but not limited to, fluidic oscillatory actuatorswhich rely on pressure oscillations to achieve self- or forcedoscillatory motion. The synchronization of the fluidic oscillators isoptionally and preferably for the purpose of obtaining a generallyuniform (e.g., within 20% or within 10% or within 5% or within 1%)oscillator frequency along the array. Optionally, the synchronizationprovides also generally uniform (e.g., within 20% or within 10% orwithin 5% or within 1%) oscillation amplitude along the array. In someembodiments, the synchronization provides also generally uniform phase(e.g., within 0.4 radians or within 0.35 radians or within 0.3 radiansor within 0.2 radians or within 0.1 radians) along the array.

The synchronization of the fluidic oscillators array according to someembodiments of the present invention is achieved by synchronizing thepressure oscillations which drive the forced oscillations. In variousexemplary embodiments of the invention a physical connection isestablished between two relevant points in each valve pair so as tosynchronize their oscillations. Optionally and preferably, suchconnection is established for each valve in the array. In someembodiments of the present invention the physical connection comprises asynchronization tube. The resistance of the tube is preferablysufficiently small such as to ensure synchronization between the valves.Sufficiently small resistance can be achieved by selecting a tube with asufficiently large cross-section. Optionally and preferably anincreasing phase lag is implemented with judicious selection of theparameters of the synchronization tube(s).

In some embodiments of the present invention the two points areconnected via a pair of connectors. When the array is large, twosynchronization conduits with multiple ports can be employed. Each suchsynchronization conduit can be connected to the appropriate location ineach valve of the synchronized pair, to form a synchronized oscillation.

In some embodiments of the present invention at least a few of (e.g.,all) the actuators are Suction and Oscillatory Blowing (SaOB) Actuators.Oscillatory-blowing is an effective tool to delay boundary layerseparation. The SaOB actuator employs steady suction and periodicblowing through openings in the surface (e.g., narrow spanwise orstreamwise slot or array of holes) to enhance shear-layer mixing andtransfer fluid from outside the shear-layer to the wall region, thuspreventing or reducing boundary layer separation.

SaOB actuators are known, and found, for example, in U.S. Pat. No.7,055,541. A representative example of an SaOB 20 suitable for thepresent embodiments is illustrated in FIGS. 1A-1F.

SaOB actuator 20 comprises an ejector member 22 characterized by a firstdiameter 24(d 1). The ejector member 22 is capable of directing a jet 26(wide white arrow) of fluid at a controlled input pressure. The fluidmay be, for example, air (gas) or water (liquid) or two or three phaseflow of gas, liquid and solid particles. SaOB 20 further comprises ajoining channel 30 characterized by a second diameter 32 (d 2). Invarious exemplary embodiments of the invention d2 is greater than d1.The joining channel 30 is in fluid communication and is capable ofreceiving flow 26 from the ejector member 22.

SaOB actuator 20 comprises one or more suction slot 34 in fluidcommunication with the joining channel 30 and an environment 36 externalto the SaOB actuator. Suction slot(s) 34 are configured for allowingadditional fluid 38 to join the jet 26 to create an amplified flow 40.

The term “slot” as used in suction slot 34 is to be construed in itswidest possible sense for purposes of this specification and theaccompanying claims. Slot, as used herein, refers to any open, oropenable, channel of fluid communication. Thus, suction slots may beeither permanent openings or openable apertures of any cross sectionalshape.

SaOB actuator 20 can further comprise a deflection device or a set ofcontrol ports 42 configured for applying a transverse pressuredifferential (41 and/or 43; cross hatched arrows) to a longitudinal axis44 of the jet 26 to direct amplified flow 40 in a first desired exitdirection 46 (FIGS. 1A, 1C and 1E). Deflection device or set of controlports 42 is also configured for redirecting the amplified flow in atleast one additional desired exit direction 48 (FIGS. 1B, 1D and 1F) bymodifying a circumferential angle by which pressure differential (41and/or 43) is transverse to longitudinal axis 44.

In FIGS. 1A-1F, a total of two exit directions 46 and 48 are illustratedbecause the circumferential angle by which pressure differential (41and/or 43) is transverse to longitudinal axis 44 has been modified by180 degrees. It will be appreciated that any total number of exitdirections 46 and 48 may be achieved by modifying the circumferentialangle by which pressure differential (41 and/or 43) is transverse tolongitudinal axis 44 by a circumferential angle defined by 360 degrees/nwhere n is the total number of exit directions 46 and 48 desired. Thus,if n=3, the circumferential angle is 120 degrees, two additional exitdirections 48 are defined and a total of three exit directions 46 and 48are employed. If n=4, the circumferential angle is 90 degrees, threeadditional exit directions 48 are defined and a total of four exitdirections 46 and 48 are employed and so on and forth.

In the pictured embodiments first desired exit direction 46 andadditional exit direction 48 are each defined by an exit port 54. Again,while two exit ports 54 are pictured, the scope of the inventionincludes mechanisms with as many as n exit ports where n is the totalnumber of exit directions 46 and 48 desired as described hereinabove.Exit ports 54 may be defined, for example, by introduction of splitter56 into conduit 30. Splitter 56 is optionally and preferably triangular(FIGS. 1A and 1B).

It will be appreciated that the total transverse pressure differentialis the vector sum of positive differential 41 directed towards axis 44and negative differential 43 directed away from axis 44. Thus, variousembodiments of the invention may employ deflection devices or controlports 42 that apply only positive differential(s) 41, that apply onlynegative differential(s) 43 or that apply both positive differential(s)41 and negative differential(s) 43.

Similarly, some preferred embodiments of the invention rely uponalternately applying only positive differential 41 and applying onlynegative differential 43 on the same side of axis 44.

Deflection device 42 may, for example, include at least one control porthaving a fluidic valve 64 (FIGS. 1A and 1B) capable of supplying atleast a portion of (e.g., 41 and/or 43) pressure differential transverseto longitudinal axis 44 of flow 26 with a predetermined periodicity.According to this embodiment transverse pressure differential 41 and 43is initially employed to direct amplified flow 40 in first exitdirection 46. In response to a command from a controller (not shown),the circumferential angle of transverse pressure differential 41 and 43is rotated by 180 degrees and amplified flow 40 is directed toadditional exit direction 48 (FIG. 1B). This process can be iterativelyrepeated in response to commands from the controller. The end result isthat amplified flow 40 oscillates between exit directions 46 and 48 at afrequency determined by the controller.

Oscillation of flow 40 can also be achieved without the use of acontroller. For example, it was found by the present inventors thatoscillations can be generated by establishing a feedback loop betweenthe control ports 42. In these embodiments, the oscillation isoptionally and preferably generated without any moving part or energyexpenditure. Such oscillation is referred to herein as self-oscillation.

In some embodiments of the present invention deflection device 42comprise at least two opposing zero-mass-flux devices (FIGS. 1C and 1D)operating at a predetermined periodicity. Each of the zero mass fluxdevices 58 is capable of supplying at least a portion (41 and/or 43) ofthe pressure differential transverse to longitudinal axis 44 of the jet26. Oscillation between exit directions 46 and 48 is achieved by causingzero mass flux devices 58 to operate out of phase so that at a firsttime point (FIG. 1C) one diaphragm 57 flexes into zero-mass-flux device58 to create a positive pressure differential 41 while the diaphragm 57of the second flexes out of zero-mass-flux device 58 to create anegative pressure differential 43. Amplified flow 40 is thus directedtowards first exit direction 46 defined by exit port 54. At a subsequenttime point, one half period of the oscillation frequency of zero massflux devices 58, the situation is reversed (FIG. 1D) and amplified flow40 is directed towards second exit direction 48 defined by exit port 54.According to additional embodiments of the invention, more than two zeromass flux devices 58 are employed to define more than two exitdirections 46 and 48. Regardless of the total number of zero mass fluxdevices 58 employed, the total transverse pressure differential is thevector sum of all partial pressure differentials 41 and 43.

Alternately, or additionally, deflection device 42 may, for example,include at least two resonance tubes 66 (FIGS. 1E and 1F). Each ofresonance tubes 66 is independently capable of capturing a portion 41 ofamplified flow 40 as it flows in one of desired exit directions 46 or 48and applying captured portion 41 of amplified flow transverse 41 tolongitudinal axis 44 of flow 26 to create pressure differential 41. Thiscauses amplified flow 40 to alter its exit direction.

The controller, if employed, may be mechanical, electronic or acombination thereof. Preferably, the controller includes a computerizeddata processing device and suitable hardware interfaces operable by thecontroller with at least a certain level of autonomy once the commandsare determined. Alternately, or additionally, the controller may requiremanual input of commands.

Preferably, suction slot(s) 34 is deployed on a surface in contact witha boundary layer of an external fluid flow 33 (FIG. 1A) so that theadditional fluid 38 joins the jet 26 via at least one suction opening 34includes at least a portion of external fluid flow 33. External, as usedwith respect to flow 33, indicates external to the SaOB actuator 20.

Optionally, but preferably, at least a portion of flow 26 emanating fromthe ejector member 22 is supplied by at least one oscillatoryzero-mass-flux jet 58 (FIGS. 1C and 1D). U.S. Pat. No. 6,751,530, thecontents of which are hereby incorporated by reference, provides detailsof the principles of operation of oscillatory zero-mass-flux jets.

Optionally and preferably, flow 26 is mixed in proximity to a junctionbetween the ejector member 22 and the joining channel 30. Mixing may beaccomplished by means of a mixer, which may rely, at least in part, uponat least one protrusion 62 from an inner surface of ejector member 22.Protrusion(s) 62 create a disturbance in the jet 26 as flow 26 passesthereupon and mixing results. Alternately, or additionally, the mixermay include an active oscillatable (mechanical or fluidic) device,capable of introducing sufficient unsteadiness to the flow such thatmixing is enhanced.

FIG. 2 is a schematic illustration of a fluidic system 200, according tosome embodiments of the present invention. System 200 comprises aplurality of fluidic oscillatory actuators. Two actuators, 202 and 204,are shown in FIG. 2 but any number of actuators can be employed.Actuators 202 and 204 are preferably unsteady fluidic actuators. Anytype of unsteady fluidic actuator is contemplated, including, withoutlimitation, mechanical piezoelectric actuator, piezoelectric fluidicactuator, pulsed combustion jet actuator, Hartmann tube, plasmaactuator, and SaOB actuator.

At least one of the actuators has an ejector member and an oscillatormember. The ejector member receives a flow 26 of fluid at a controlledinput pressure and provides an amplified flow 40 (for example, by meansof additional fluid, not shown in FIG. 2, entrained flow via one or moresuction slot(s), as further detailed hereinabove). The oscillator memberreceives the amplified flow 40 and generates the fluidic oscillations(for example, by means of a deflection device or controlled ports 42 asfurther detailed hereinabove). For clarity of presentation, only theoscillator members are illustrated in FIG. 2. Ejector members suitableto some embodiments of the present invention are illustrated in FIGS.1A-1F and 8A-8C.

System, 200 comprises one or more synchronization conduit 206 connectingat least two of the actuators such as to effect synchronization betweenthe oscillations in the respective actuators. For example, when theactuators are SaOB actuators, conduit 206 can be connected between therespective deflection devices 42 of the actuators.

In the representative illustration of FIG. 2, which is not to beconsidered as limiting, synchronization conduits are connected at boththe deflection devices (designated control port A and control port B) ofeach connector such that control ports of the same side are connected(port A to port A and port B to port B). Also contemplated areembodiments in which control ports of opposite sides are connected bythe synchronization conduit (port A to port B). Such configuration canbe used for generating opposite oscillations (about 180 degreesout-of-phase) between a pair of actuators. The pressures differencesalong synchronization conduits 206 are shown in FIG. 2 using colorcodes, where blue corresponds to low pressures and red corresponds tohigh pressures.

In various exemplary embodiments of the invention system 200 comprisesone or more self-feedback conduits 208. Each self-feedback conduit 208is constituted to establish oscillatory pressure gradient between thetwo control ports of the same actuator.

In some embodiments of the present invention each control port of eachoscillatory actuator is respectively connected to at least two controlports of at least two another oscillatory actuators. In alternativeembodiments, each control port of each oscillatory actuator is connectedto only one control port of another oscillatory actuator. Otherconfigurations or combination of configurations are not excluded fromthe scope of the present invention.

It is recognized by the present inventors that synchronization betweenthe actuators reduces the drag. In an experiment performed by thepresent inventors a 1:12 scaled truck model was placed in a wind tunnel.FIG. 3 shows the form drag coefficient as a function of the actuators'inlet pressure as measured during the experiment. The red round symbolscorrespond to synchronized mode of operation and the magenta squaresymbols correspond to an unsynchronized mode of operation. As shown,there is larger drag reduction in the synchronized mode of operationthan in the unsynchronized mode of operation. For example, at an inletchannel supply pressure of 0.023 MPa, the technique of the presentembodiments doubles the drug reduction. Further details regarding theexperiment are provided in the Examples section that follows.

Optionally, a controlled phase lag is generated. This can be done byincreasing the resistance of or decreasing the cross section of thesynchronization tubes/ducts. FIG. 4 shows results of increasing thephase lag between two valves by reducing the cross section of thesynchronization tube. Shown in FIG. 4 are the relative phase (blue) andfrequency variation (red) for 3 different synchronization channelcross-section areas.

The system of present embodiments can be used in many applications. Insome embodiments the system is used for actively controlling the flow atboundary fluid layers, in some embodiments the system is used foractively controlling the flow over a bluff body, in some embodiments thesystem is used for actively controlling wake flow, in some embodimentsthe system is used for actively controlling lift (e.g., of an aerialvehicle), in some embodiments the system is used for activelycontrolling moment (e.g., of a rocket or aircraft), and in someembodiments the system is used for actively reducing drag. The system ofthe present embodiments is particularly useful for controlling one ormore of the above quantities at the surface of a vehicle, such as, butnot limited to, a truck aft-body, a shipping container on a truck bed, asemi-trailer, a trailer or an aeronautical vehicle such as a rocket oran aircraft helicopters, external stores or the like. The system of thepresent embodiments is also useful for controlling one or more of theabove quantities at various types of airfoils, including, withoutlimitation, a wing of an aeronautical or ground vehicle or a blade of awind turbine.

The present embodiments can be implemented as an active separationcontrol system, such as an AFC system for alter flow behavior usingsmall, unsteady, localized energy injection or fluid removal. In someembodiments of the present invention system 200 is used for reducingaerodynamic drag. Aerodynamic drag is the cause for more than two-thirdsof the fuel consumption of large trucks at highway speeds (e.g., U.S.permitted highway speeds). Due to functionality considerations, theaerodynamic efficiency of the aft regions of large trucks wastraditionally sacrificed. This leads to massively separated flow at thelee side of truck trailers, with an associated drag penalty: roughly athird of the total aerodynamic drag. In various exemplary embodiments ofthe invention an AFC system, which may be or comprise system 200, isattached to the back side of a vehicle, such as, but not limited to, atruck trailer or an aeronautical vehicle. The AFC system of the presentembodiments redirects the flow separating from the vehicle to turn intothe lee side of the vehicle thereby increasing the base pressure (staticpressure on the lee-side) and significantly reducing drag.

FIG. 5 is a schematic representation in a lateral cross sectional viewof system 200 according to an exemplary embodiment of the inventioninstalled on a blunt-body 180. In an exemplary embodiment of theinvention, blunt body 180 is a truck aft-body or a shipping container ona truck bed, semi-trailer or trailer. In the depicted embodiment, system200 is installed on upper and lower parts of trailing face 184.Alternatively, or additionally, system 200 can be installed on lateral(vertical) edges of trailing face 184.

Air flows around blunt body 180 as it travels forward (left in thisview) with leading face 182 (seen as an edge in this view) disruptingthe airflow so that an external flow is created above upper surface 186of blunt body 180, along the sides of body 180, and below lower surface188 of blunt body 180. Typically, but not necessarily, all externalflows separate and continue to flow parallel beyond trailing face 184 ofblunt body 180 at the height of upper surface 186 and lower surface 188,respectively and from the sides of body 180.

In the exemplary embodiment of FIG. 5, which is not to be considered aslimiting, system 200 comprises two actuator chambers, each optionallyand preferably comprises an array of synchronized actuators. However,this need not necessarily be the case, since, for some applications, itmay not be necessary for the system to include two chambers. Forexample, a single chamber or more than two chambers can be employed.Each chamber is depicted as having a 180 degree circular arc in crosssection. Optionally, arc angles of 90 degrees or less are sufficient interms of flow control functionality and are used to merely attach system200 to trailing face 184.

Physical presence of the actuator at the upper part of face 184according to an embodiment of the invention causes passive redirectionof external flow layer airflow in a downward direction as indicated by136. An actuator mounted adjacent to lower edge 188 according to someembodiments of the present invention causes a similar passiveredirection of external airflow in an upward direction as indicated by138. This effect can be attributed to the function of system 200 as apassive deflector. The same principle holds when system 200 is mountedon to the sides of the blunt surface 184.

It is recognized by the present inventors that when an active separationcontrol system is provided as an add-on system to an existing vehicle,particularly a vehicle having a rear door, it can cause a conflict withthe ability of the rear door to open beyond 180 degrees. To allowopening the rear door beyond 180 degrees, the active separation controlsystem can be mounted on the door in a detachable manner, so that thesystem is dismounted before opening the door and remounted once the dooris closed.

Alternatively, an active separation control system according to someembodiments of the present invention can be constructed to allow openingthe rear door beyond 180 degrees without dismounting the system from therear door. This can be done by providing a system, which comprises afluidic oscillatory actuator having an ejector member and an oscillatormember both mounted on one or more flexible member. The fluidicoscillatory actuator is mountable on a rotatable door of a vehicle suchthat the flexible member is more curved when the door is closed thanwhen the door is open.

A representative example of such configuration according to someembodiments of the present invention is illustrated in FIGS. 6A and 6B.Shown in FIGS. 6A-6B is an active separation control system 260 with afluidic oscillatory actuator 270 having an ejector member 268 and anoscillator member 266. System 260 can be similar to system 200 above, orit can be of any other type suitable for providing fluid actuation inthe form of unsteady flow. For example, system 260 can be similar to thesystem described in U.S. Published Application No. 20100194142 orInternational Publication No. WO WO2011/077424, the contents of whichare hereby incorporated by reference.

System 260 is mounted on a rear door 272 of a vehicle, such as a traileror the like. For clarity of presentation only a side wall 274 of thevehicle is illustrated. Door 272 is connected to a rotatable bar 276which is allowed to rotate over a range of more than 180 degrees, e.g.,200 degrees, or 220 degrees, or 240 degrees or more. FIG. 6A illustratessystem 260 when the door is closed, and FIG. 6B illustrates system 260when the door is open.

The flexibility of system 260 is optionally and preferably achieved bymeans of a flexible sheet (e.g., a metal sheet) 262 which form a contourin the shape of the active separation control system (e.g., a section ofa cylinder or a cylinder, shown an arc in FIG. 6A). In various exemplaryembodiments of the invention the ejector member is attached at the innerpart of one flexible sheet member and the oscillator member is attachedat the inner part of the other flexible sheet member. The ejector memberand oscillator member can be connected via a flexible tube 280 whichserves as a joining channel. When system 260 comprises a feedback and/orsynchronization conduit (not shown, see FIG. 2), these conduits arepreferably flexible.

Optionally, a cable 282 connects the end of the sheet 264 to the door272 so as to maintain the location of system 269 fixed against the door.When the door is closed (FIG. 6A), the cable becomes tight and theflexible sheet assumes the required shape. Opening the door releases thetension on the cable, so that the flexible sheet becomes less curved(e.g., generally flat) and is shifted closer to the door, so that thecable becomes loose (FIG. 6B).

The present inventors contemplate several configurations for enhancingthe efficiency and/or compliance of system 20. The Examples section thatfollows provides several efficiency enhancement considerations. As arepresentative example, the frequency of the oscillatory actuators isoptionally and preferably X times (e.g., twice) the expected vortexshedding frequency of fluid at the vicinity of system 200, wherein X isfrom about 1.5 to about 3.5 more preferably from about 1.8 to about 3.2.

As another representative Example, the separation between adjacentfluidic oscillatory actuators is about X·v·St/f, wherein v is theexpected velocity of fluid at the vicinity of the system, f is anexpected vortex shedding frequency of fluid at the vicinity of thesystem, St is the expected Strouhal number characterizing the fluidicoscillations, and X is a dimensionless number from about 0.5 to about 4or from about 1 to about 3 or from about 1 to about 2.

It is expected that during the life of a patent maturing from thisapplication many relevant unsteady fluidic actuators will be developedand the scope of this term is intended to include all such newtechnologies a priori.

As used herein the term “about” refers to ±10%.

The word “exemplary” is used herein to mean “serving as an example,instance or illustration.” Any embodiment described as “exemplary” isnot necessarily to be construed as preferred or advantageous over otherembodiments and/or to exclude the incorporation of features from otherembodiments.

The word “optionally” is used herein to mean “is provided in someembodiments and not provided in other embodiments.” Any particularembodiment of the invention may include a plurality of “optional”features unless such features conflict.

The terms “comprises”, “comprising”, “includes”, “including”, “having”and their conjugates mean “including but not limited to”.

The term “consisting of” means “including and limited to”.

The term “consisting essentially of” means that the composition, methodor structure may include additional ingredients, steps and/or parts, butonly if the additional ingredients, steps and/or parts do not materiallyalter the basic and novel characteristics of the claimed composition,method or structure.

As used herein, the singular form “a”, “an” and “the” include pluralreferences unless the context clearly dictates otherwise. For example,the term “a compound” or “at least one compound” may include a pluralityof compounds, including mixtures thereof.

Throughout this application, various embodiments of this invention maybe presented in a range format. It should be understood that thedescription in range format is merely for convenience and brevity andshould not be construed as an inflexible limitation on the scope of theinvention. Accordingly, the description of a range should be consideredto have specifically disclosed all the possible subranges as well asindividual numerical values within that range. For example, descriptionof a range such as from 1 to 6 should be considered to have specificallydisclosed subranges such as from 1 to 3, from 1 to 4, from 1 to 5, from2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numberswithin that range, for example, 1, 2, 3, 4, 5, and 6. This appliesregardless of the breadth of the range.

Whenever a numerical range is indicated herein, it is meant to includeany cited numeral (fractional or integral) within the indicated range.The phrases “ranging/ranges between” a first indicate number and asecond indicate number and “ranging/ranges from” a first indicate number“to” a second indicate number are used herein interchangeably and aremeant to include the first and second indicated numbers and all thefractional and integral numerals therebetween.

It is appreciated that certain features of the invention, which are, forclarity, described in the context of separate embodiments, may also beprovided in combination in a single embodiment. Conversely, variousfeatures of the invention, which are, for brevity, described in thecontext of a single embodiment, may also be provided separately or inany suitable subcombination or as suitable in any other describedembodiment of the invention. Certain features described in the contextof various embodiments are not to be considered essential features ofthose embodiments, unless the embodiment is inoperative without thoseelements.

Various embodiments and aspects of the present invention as delineatedhereinabove and as claimed in the claims section below find experimentalsupport in the following examples.

EXAMPLES

Reference is now made to the following examples, which together with theabove descriptions illustrate some embodiments of the invention in a nonlimiting fashion.

Example 1 Drag Reduction Using Active Flow Control

This Example describes an active separation control system, packaged asan add-on device attachable to the rear end of a vehicle, such as alarge truck trailer. The system can reduce the aerodynamic drag by about20%.

Following is a description of a comprehensive study that included, incombination, actuator development, computational fluid dynamics andbench-top, as well as wind tunnel testing. The study employed an arrayof 15 newly developed suction and oscillatory blowing actuators housedinside a circular cylinder attached to the aft edges of a generic 2Dtruck model.

Introduction

AFC is a fast-growing, multidisciplinary science and technology thrustaimed at altering a natural flow state to a more desired flow state (orpath). Flow control was simultaneously introduced with the boundarylayer concept at the turn of the 20th century. Though the fluidmechanics aspect can be robust, steady-state flow control methods wereproven to be of inherently marginal power efficiency, and thereforelimited the implementation of the resulting systems. Unsteady flowcontrol using periodic excitation and utilizing flow instabilityphenomena (such as the control of flow separation [2]) has the potentialof overcoming the efficiency barrier. Separation control using periodicexcitation at a reduced frequency of the same order, but higher than thenatural vortex shedding frequency of bluff bodies (such as an airfoil inpost-stall or a circular cylinder), can save 90% to 99% of the momentumrequired to obtain similar gains in performance compared to theclassical method of steady tangential blowing. The present embodimentsare useful for increasing the efficiency and simplifying fluid relatedsystems. When applied, e.g., to a fleet of trucks, these embodiments canallow saving in fuel consumption, which is advantageous from thestandpoints of economy and environmental considerations.

The progress in the development of actuators, sensors, simulationtechniques and system integration and miniaturization enables using widebandwidth unsteady flow control methods in a closed-loop AFC (CLAFC)manner. (See e.g., Ref. [3] for a comprehensive review of the subject.)Experimental demonstrations are required to close the gap between thecurrent theoretical understanding, the computational capabilities andreal-world problems. The study described in the present example bringstogether AFC expertise, specifically actuator development andimplementation for separation control, closer to real-life industrialapplications.

At highway speeds, the aerodynamic drag of a vehicle is responsible forroughly 65% of the fuel consumption, making the potential fuel savingsabout 10%, taking into account the energy cost of the AFC system. Therehas been considerable effort in the US to reduce the fuel consumption oftrucks using shape changes, simple add-on devices and steady state AFCmethods [4]. It is recognized by the present inventors that thosetechniques have inherent limitations and are insufficient. Reference [4]cites several research efforts focused on truck trailer drag reduction.Adjustable inclination flat plates are attached at the truck lee side.These plates, however, are rather large, heavy and expensive, and theirsize raises functionality and compatibility issues. Periodic excitationwas also mentioned in Ref [4]. Steady blowing [10] was also applied to amodified aft region of a truck trailer and resulted in significantaerodynamic drag reduction, but at a marginal to zero or mostly negativeenergy efficiency due to reasons identified in [2].

The study described below was aimed at applying AFC technology as an“add-on” device attached to the aft body of the vehicle. Suction andOscillatory Blowing (SaOB) AFC actuators are used for drag reduction ofheavy ground transportation systems. The above fluidic device is acombination of an ejector (jet-pump) and a bi-stable fluidic amplifier[Arwatz et al (2007)]. The current study was assisted by a computationalfluid dynamics (CFD) to narrow the huge parameter space. Aftercompleting the actuator development and adaptation to the speed rangerelevant to trucks, three stages of experiments were performed on acircular cylinder, the generic bluff-body. These studies resulted in asignificant reduction of drag due to delay of boundary layer separation.

A wide range of boundary conditions were tested and only the commonresults to all conditions were considered as valid. Successful windtunnel demonstration on a two-dimensional (2D) equivalent of a blunttruck trailer model was subsequently performed. The technology of thepresent embodiments successfully provides aerodynamic drag reduction,particularly of heavy road vehicles and aeronautical systems.

Description of the Experiment

The wind tunnel experiments were performed on a generic 2D equivalentmodel of a large truck, along the lines of the GTS model [15]. FIG. 7presents a cross section of the model, showing also the location of the43 pressure taps.

The model height (H) is 450 mm and its length is 1250 mm. It spans theentire width of the wind tunnel test section, b=609 mm. The model ismade of aluminum beams and ribs and a skin of 2 mm thick aluminumplates. The significant interference with the wind tunnel has not beentaken into account in any manner.

During the 2D truck experiments the ceiling static pressures weremeasured and could be used for evaluation of the wind tunnelinterference with the use of future CFD effort. It is argued that sincethe flow accelerates more around the model, with respect to free flowconditions, the AFC results are conservative since the AFC effects areproportional to the flow linear momentum at the separation points. Forcontrol purposes, one or two circular cylinders were attached to the aftregion of the truck model. The cylinders were 76.2 mm in diameter andspanned the 609 mm of the wind tunnel. The cylinders were installed sothat they were tangent to the aft body corners and extended one radiusbehind the aft plate line, their centers one radius below the uppercover plate or above the lower cover plate, respectively. See FIG. 13Awith only the upper control cylinder sketched. The upper controlcylinder was installed with an array of 15 SaOB actuators. An array of96 suction holes with diameter of 2 mm and spacing of 6 mm were drilledin the cylinder. The wall thickness was close to 10 mm, so the flowresistance was high. Fraction of the outer flow was sucked into thecylinder due to the sub pressure created by the SaOB actuator array. Theentire flow rate (the sum of the inlet and entrained-sucked flow) wasejected alternatively out of two tangential, 1.7 mm high, pulsed blowingslots connected to each actuator. Each actuator was 28 mm wide and itcontrolled about 40 mm of the span of the cylinder. The lower controlcylinder was used only for steady-suction and was connected to anexternal suction pump. It had two staggered rows of 96 holes 2 mmdiameter each, spaced 7.5 deg apart along the cylinder arc (FIGS. 16Aand 16B).

A simulated road was placed under the 2D truck in some of the tests. Theplate was positioned 67 cm upstream of the model and was 280 cm long, 4mm thick, extending 90 cm behind the model. A 3D wake rake waspositioned 120 cm downstream of the model. It measured 29 totalpressures and 2 static pressures, one close to each sidewall. The wakerake was mounted on a Y-axis traverse allowing vertical motion, withtypical resolution of 20-25 mm.

The experiments were performed in the Meadow-Knapp low-speed windtunnel. The speed range is 4-60 m/s, the turbulence level is about 0.1%and the test section dimensions are 1.5 m (high) by 0.61 m (wide) and4.25 m long. Pressures were measured by a PSI Inc. pressure scanner with128 ports at a resolution of 0.001 psi. Tunnel dynamic pressure wasmeasured by a Pitot-Prandtl tube, positioned 110 cm upstream of themodel leading edge on the tunnel ceiling, connected to a Mensor pressuretransducer (10″ water full scale and resolution of 0.06%). The Reynoldsnumber was monitored to 1% tolerance and unsteady pressures were alsomeasured on the model aft region, to identify unsteady effects. Thetunnel temperature was 24±2° C. Air density and viscosity werecalculated using standard formulae and the ambient temperature andpressure.

Suction and Oscillatory Blowing (SaOB) Actuator

The SaOB actuator is described in U.S. Pat. No. 7,055,541. Several sizeactuators were developed. A theoretical model for the valve operationwas validated (Arwatz et al, 2007). The actuator is a combination of abi-stable fluidic oscillator member connected downstream of an ejectormember. A simplified isometric view of the actuator is shown in FIG. 8Aand schematic illustrations explaining the principle of operation of theejector and oscillator are shown in FIGS. 8B and 8C, respectively. Thepurpose of the ejector is to create a suction flow, by amplifying theflow entrained into the valve. It has been shown that the ejector isindeed increasing the flow rate by a factor up to three with itsentrances unrestricted. To create self-oscillations, the two controlports were connected by a short tube, without any moving part or energyexpenditure. The tube was later replaced by an S-shaped channel machinedin a plate on which all the actuators were installed (referred hereafteras “mounting plate”). The SaOB actuator has a wide and appropriatefrequency range (0.1 to 1.4 kHz) depending on the ejector nozzle shape,the length of the feedback tube and the inlet flow-rate (Arwatz et al,2007). Near-sonic actuator exit velocities have been measured, butcurrently the exit velocities are of the same order as the free-streamvelocities. The inlet flow rate is controlled by a pressure regulatorconnected to shop air supply—to be replaced by the truck pneumaticsystem—by flow extracted from the truck turbocharger or by an auxiliarysystem connected electrically to the truck alternator or mechanically tothe rear wheels. The valve operation is insensitive to rain or dustconditions.

To fit the valve into the cylinder while minimizing the pressure dropacross it, the ejector nozzle was designed to have a short convergingstraight geometry and the mixing chamber between the ejector and theoscillator were shortened (FIG. 8A). Following these modifications thevalve was bench-top tested, initially only with exit restrictions,simulating the assembly in the control cylinder.

FIGS. 9A-9B show the maximum and minimum flow velocity out of a singleSaOB actuator with an exit assembly simulating the conditions that willprevail in the “add-on” device as well as in the control cylinder athalf scale. The exit velocities were measured by a hot wire that wastraversed along the exit slot. The cross-section of the feedback tubes(all with L=80 mm) was altered during those tests. As shown, the crosssection of the feedback tube has a strong effect on the oscillationfrequency. It has a weaker effect on the switching quality κ, definedas:

$\kappa = {\frac{U_{\max} - U_{\min}}{U_{average}}.}$

Circular Cylinder with the SaOB Actuator Array

The 15-valve array was mounted on a plate, providing inlet pressure toall the valves, feedback for each valve self-oscillation, andsynchronization tubes between the valves. The cross section of thefeedback tubes was 5.7 mm² and their length was maintained at 80 mm.FIGS. 10A and 10B show the velocity (FIG. 10A) and frequency (FIG. 10B)of oscillation of an actuator array installed in the circular cylinderand tested on the bench-top set-up. The valves were not allsynchronized, the frequencies of the central 13 valves deviated by nomore than 10%, and the peak velocities deviated by a maximum of 20%.

It has been established that the drag reducing capability ofsteady-suction through a 2D slot and later a row of holes in a widerange of boundary conditions. It has been also established that a 15 degdelay in separation region on the circular cylinder at Re=100,000 andRe=150,000 (associated with the target highway driving speeds for thecurrent cylinder diameter) is possible with suction magnitude of abouthalf the free-stream velocity. Re at 90 km per hour is about 200,000 forthe device diameter as a char length. This enabled the definition of theconfiguration shown in FIG. 11A, which is an illustration of a crosssection of the 15-valves actuator array as installed inside thecylinder.

The suction holes are located 15 deg upstream of the pulsed blowingslot. The array of 15 SaOB actuators is mounted inside the 76.2 mmdiameter cylinder. Inlet flow is provided via common channel feeding allthe ejectors' jets. These create low pressure in the half-cylinder tothe left of the valve array, sucking flow through the holes. The entireflow is then ejected through the pulsed blowing exit slots. Theoscillation frequencies are in most cases larger than the natural vortexshedding frequencies on a circular cylinder at the current velocityrange and significantly higher than the 2D truck model vortex sheddingfrequency.

Three series of wind tunnel experiments were performed on the circularcylinder. FIG. 11B is a drawing of an image of the experimental set-upincluding the cylinder in the Meadow-Knapp wind tunnel.

The cylinder was tested and a sample of the drag reduction data ispresented in FIGS. 12A-12B. The data shown in FIGS. 12A-12B indicates adrag coefficient reduction from about 1.1 to about 0.7, or a relativedrag reduction of about 35%. This is obtained when the suction holes arelocated at about 110 deg (relative to the free-stream direction) on thecylinder and the pulsed blowing slot at are positioned 15 deg downstream(at about 125 deg), where 90 deg is the summit of the cylinder. Theenergy efficiency of this drag saving, which was measured at flow speedcomparable to the highway speed of trucks, indicates net positiveenergetic efficiency.

Two-Dimensional (2D) Truck Experiments

Two-Dimensional (2D) Truck with SaOB Cylinder at Upper Aft Corner

FIGS. 13A-13D show a 2D truck model with an SaOB cylinder installed onits upper aft corner according to some embodiments of the presentinvention. FIG. 13A is a schematic illustration showing a cross-sectionview and incoming flow U_(∞) from left to right, FIG. 13B is a drawingof an image showing a front view of the model once installed in aMeadow-Knapp tunnel 50-55 mm above the simulated road, FIG. 13C is adrawing of an image showing a rear-view of the model including the backplate and SaOB instrumented cylinder on the top-rear end, and FIG. 13Dis a magnification of the white dashed rectangle of FIG. 13C.

The vertical distance between the model and the floor was allowed toincrease from 50 to 55 mm in order to compensate for boundary layergrowth. It was validated that the boundary layers did not restrict thefree flow under the model. Wheels or a moving floor were not used. Theseeffects seem secondary since most of the effort was spent on the upperaft corner, least effected by the wheels or simulated road, moving orstationary. However, the mere presence of the simulated road allowedsimulating a side view of the truck driving on a road. Several 50 mmwide roughness strips (grit #60) were placed on the top and bottomplates of the 2D truck model, to reduce Reynolds number effects.

An array of 96, 2 mm diameter, suction holes was positioned 15 degupstream of a 1.7 mm (nominally) pulsed blowing wide slot on thecylinder that was allowed to rotate for optimal positioning of theactuation locations (FIG. 13D). The slots allowed almost tangentialdownstream-directed introduction of the pulsed blowing excitation. Eachactuator controlled a span of about 40 mm with two exits, as shownschematically in the FIG. 9B. The trailing edge of the top and bottomcover plates were machined to create a back-step no thicker than 0.5 mm,allowing a smooth transition of the flow from the covers to the controlcylinders.

The add-on device according to some embodiments of the present inventionhas a shape that resembles a semi- to quarter-cylinder attached to theback side of a truck trailer. The advantage of having a completecylinder at the experimental stage is the capability to convenientlyalter the actuation location, which is shown to be very sensitive on thefree cylinder (FIGS. 12A-12B) and also in the results related to trucks,presented below.

At this stage of the investigation the angular distance between thesuction holes and pulsed blowing slots was fixed, 15 deg.

The drag of the 2D truck model was calculated from the integration ofthe pressures around the model and from a 3D wake survey performed 1.2 mbehind the model. The wake flow was found to be reasonably 2D and theagreement between the two methods of drag evaluation was better than 2%in most cases, and in the absence of the simulated road. The dragreduction magnitudes were similar when evaluated either with the wakeintegration method or from the pressure drag. The aft body “add-on”device was tested as a circular cylinder, due to the larger size andease of installation of the 15 SaOB valve array inside it. A secondarymajor consideration was the capability to rotate the cylinder, bringingthe holes/slot to an optimal location.

FIG. 14A shows the effect of the SaOB actuation on the drag of the 2Dtruck model at U=25 m/s for different actuation levels indicated by theinlet pressures. The actuation location is altered via cylinderrotation, where 90 deg is the cylinder-upper plate junction.

FIG. 14B shows the effect of the SaOB actuation on the required power topropel the 2D truck model at U=25 m/s for different actuation levelsindicated by the inlet pressures. The actuation location is altered viacylinder rotation, where 90 deg is the cylinder-upper plate junction.Reference power is about 2.57 kWatt.

The addition of the passive control cylinder at the upper aft corner hada drag reducing effect of about 5% with respect to the baselineconfiguration shown in FIG. 7. The slot was just exposed when itslocation was 90 deg. With the control cylinder present but when the slotwas hidden, Cdp=0.99±0.01. The passive effect of the slot, and itsassociated discontinuity, can be seen by the drag increase between 90and 100 deg. At larger slot locations the drag returns to itsundisturbed value, with a possible drag penalty of 0.01-0.02. At slotpositions greater than 105 deg both slot and suction holes are exposed.FIGS. 14A-14B show results of different levels of control applied by thearray of SaOB actuators, with increasing level of input pressures, asindicated in the legend. A significant drag reduction develops in therange of tested pressures, up to 0.05 MPa. An optimal holes/slotlocation can be identified around 130-132.5 deg, slightly increasingwith the magnitude of the control authority. These results were obtainedwith a simulated road, similar to a truck with control applied only fromthe top aft edge of the trailer. Optionally, a second control cylinderat the lower aft edge with simulated road can be added.

Following are energy cost considerations. The net power saving wascalculated according to: Power saved=0.5ρU_(∞) ³SΔC_(d)−P_(i)Q_(i)/η.Where ρ is the air density, U_(∞)=25 m/s is the free-stream velocity,S=0.61×0.45 m² is the 2D truck cross-section area, ΔC_(d) is the dragreduction at the same holes/slot position with respect to the baselineuncontrolled condition. The control power was taken as the product ofthe inlet pressure (Pi, measured at the supply line) and the inlet flowrate (Qi, measured by an orifice flow meter at the pressure regulatorand neglecting the effect of the larger static pressure on the flowrate, making the actual flow rates smaller by 5-20% than those currentlycited depending on the excess pressure). The pumping efficiency, η, wastaken as 75%, as in many common low-pressure compressors. The controlflow was provided by the lab shop air through a computer controlledpressure regulator. The data presented in FIG. 14B shows a ratherinsensitive (to the inlet pressure) peak power saving of around 130watts for inlet pressures of 0.02-0.025 MPa. With either lower or higherpressure levels, the power efficiency decreases.

The data presented in FIGS. 14A-14B were obtained at a fixed free-streamvelocity of 25 m/s.

FIG. 15A shows the effect of the SaOB actuation on the baseline andcontrolled drag of the 2D truck model as a function of the heightReynolds number for fixed actuation level. The actuation location is:pulsed blowing slot at α=130°, suction holes at α=115°, Pin=0.025 MPa,Qin=2.8 Lit/s where 90 deg is the cylinder upper plate junction.

The simulated road was present in this experiment. The baseline dragslightly increases, from 0.98 to 1.02 with Re (based on the model heightand the free-stream velocity) increasing from about 200,000 to about1,000,000. Note that at the larger Re range the drag reaches a plateau.This Re, based on the height of the model, is considered minimal forReynolds number free results. With fixed level of inlet pressure andfixed actuation locations a significant drag reduction over the entireRe range was observed. It is expected that the magnitude of the dragreduction decreases as Re increases, due to the relative decreasebetween both the suction and pulsed blowing magnitudes when normalizedby the free-stream velocity. Aerodynamic drag reduction of about 20% ispossible at low speeds, decreasing to about 5% at the highest speedsconsidered operational and legal for large trucks in the US highwaysystem.

FIG. 15B shows the net flow power saved and equivalent “fuel” saving ofthe controlled 2D truck model as a function of the driving speed forfixed actuation level. The actuation location is: pulsed blowing slot atα=130°, suction holes at α=115°, Pin=0.025 MPa, Qin=2.8 Lit/s where 90deg is the cylinder upper plate junction.

The net power saving is equivalent to about twice the expected fuelsaving after considering friction resistance and wind averagedperformance. One can note a 45-watt power saving at 45 MPH increasing to180 watts saved at 75 MPH. These power savings translate to more than a3.5% power saving at speeds between of 45-55 MPH. At higher speeds theaerodynamic power saving (taking into account the invested power in theactuation system) saturates at about 2.8%. Considering that at thesespeeds (above 60 MPH) two thirds of the power is invested in overcomingaerodynamic drag, the equivalent net fuel savings is about 1.9%. It isrecognized by the present inventors that these numbers are significant.The inlet pressure was optimal at about 50 MPH, so larger controlauthority shifts that peak to higher speeds, depending on the targetspeed range. Furthermore, the obtainable drag reduction due to theapplication of the control on the upper aft edge with the simulated roadpresent is smaller compared to its application on the sides of thetruck, as will become clear from the subsequent discussion.

Two-Dimensional Truck with Two Control Cylinders

FIG. 16A is a schematic illustration of the 2D truck model with twocontrol cylinders, and FIG. 16B is a drawing of an image showing aclose-up rear-view of the lower aft-corner control cylinder (see “viewI” in FIG. 16A).

The actuation locations are: pulsed blowing slot at α=130°, suctionholes at α=115°, Pin=0.04 MPa, where 90 deg is the cylinder summit.Lower cylinder: two rows of suction holes, the 1^(st) at α=−121°, thesecond at α=128.5°. Lower cylinder suction magnitude was tuned toprovide the same drag reduction as the SaOB array alone at Re˜800,000.

The lower control cylinder was also mounted on a rotary system to allowoptimal control locations. Only steady-suction was applied at the lowercontrol cylinder. To increase efficiency, SaOB actuation can beemployed. The suction was applied from two rows of staggered 2 mmdiameter holes, each containing 96 holes and spaced 7.5 deg in theirangular locations. The lower suction holes were just exposed for holeslocation of −90 deg. The same optimization procedure was applied to thelower cylinder, as previously applied to the upper control cylinder, inorder to identify a condition which provides the same level of dragreduction that the upper SaOB control cylinder is capable of with inletpressure of 0.04 MPa. This higher pressure level was selected based onthe results shown in FIGS. 14A-14B and 15A-15B and discussed above.

FIG. 16C shows drag reduction due to SaOB array on top aft, Suctioncylinder on bottom aft corners. Shown is form-drag coefficient as afunction of Reynolds number for four states: baseline, only the upperSaOB cylinder is turn on, only the lower suction cylinder is on, andboth control cylinders are on. The data presented shows that thebaseline drag of the 2D truck model, with two control cylinders andwithout the simulated road is 0.93±0.01 regardless of the Reynoldsnumber. The two control cylinders, when operated alone, can providesignificant and similar drag reduction over the entire Re range, withthe exception of the SaOB control that is more effective at low speeds.This difference might be associated with the oscillation frequency beingsomewhat low, and therefore optimal at low speeds. It was surprisinglyfound that when the two control cylinders operated together, the dragreducing effects accumulate. The data presented demonstrates about 20%aerodynamic drag reduction at highway speeds, translated to about 10%net fuel savings on large trucks, busses, and tractor trailerconfigurations.

It was found by the present inventors that a preferred location forintroducing the suction through holes is about 15-20 deg downstream ofthe plate-cylinder junction. During the cylinder-alone tests, it wasfound that suction with half the free-stream magnitude is capable of 15deg separation delay. Therefore, the pulsed blowing was introduced 15deg further downstream of the suction holes. One control cylinderpositioned at the upper aft edge of the simulated trailer is capable ofabout 10% drag reduction. But if the power invested in the actuation isconsidered, the optimum is obtained at lower fluidic power input, wherethe aerodynamic drag reduction is roughly 6-7%. When two controlcylinders were applied in a situation simulating a control applied tothe two vertical edges of the aft-trailer region, a 20% drag reductionis possible. This should lead to at least 10% net fuel savings on a fullscale truck.

It is expected that for a full scale vehicle there will be a significantenhancement in power efficiency. This enhancement will originate fromlower resistance of the suction holes (due to larger diameter, smallerwall thickness and rounded edges) and a factor of 2-4 power saving onthe actuators' ejector due to the larger ejector nozzle cross section.

REFERENCES

-   1. Prandtl, L., “Motion of Fluids with Very Little Viscosity”, Third    International Congress of Mathematicians at Heidelberg, 1904, from    Vier Abhandlungen zur Hydro-dynamik and Aerodynamik”, pp. 1-8,    Gottingen, 1927, NACA TM-452, March 1928.-   2. Seifert, A., Darabi, A. and Wygnanski, I., 1996, “Delay of    Airfoil Stall by Periodic Excitation”, J. of Aircraft. Vol. 33, No.    4, pp. 691-699.-   3. Collis, S. S., Joslin, R. D, Seifert, A. and Theofilis, V.,    “Issues in active flow control: theory, simulation and experiment”,    Prog. Aero Sci., V40, N4-5, May-July 2004 (previously AIAA paper    2002-3277).-   4. Annual Progress Report for Heavy Vehicle System Optimization,    February 2005, US Dep of Energy.-   5. Seifert, A., Paster, S., “Method and mechanism for producing    suction and periodic flow”, U.S. Pat. No. 7,055,541.-   6. Arwatz, G., Fono, I. and Seifert, A., “Suction and Oscillatory    Blowing Actuator”, paper presented in the MEMS IUTAM meeting,    September 2006, London, UK.-   7. Pack Melton, L. G., Schaeffler, N., Yao, C. S. and Seifert, A.,    “Active Control of Flow Separation from the Slat Shoulder of a    Supercritical Airfoil”, J. of Aircraft, 42 (5): 1142-1149    September-October 2005 (previously AIAA Paper 2002-3156).-   8. Pack Melton, L. G., Yao, C. S. and Seifert, A., “Active Control    of Flow Separation from the Flap of a Supercritical Airfoil”, AIAA    J., 44 (1): 34-41 Jan. 2006 (previously AIAA Paper 2003-4005).-   9. Seifert, A. and Pack, L. G., “Active Control of Separated Flow on    a Wall-mounted “Hump” at High Reynolds Numbers”, AIAA J., V. 40, No.    7, July, 2002, pp. 1363-1372. (Part of AIAA paper 99-3430).-   10. Englar, R. J., Advanced aerodynamic device to improve the    performance, economics, handling and safety of heavy vehicles, SAE    paper 2001-01-2072-   11. Englar, R. J., Pneumatic Aerodynamic control and drag reduction    system for ground vehicles, U.S. Pat. No. 5,908,217, Jun. 1, 1999.-   12. Arwatz, G., Fono, I. and Seifert, A. “Suction and oscillatory    Blowing Actuator”, AIAA J. 2007, to appear.-   13. R. C. McCallen, K. Salari, J. Ortega, L. DeChant, B. Hassan, C.    Roy, W. D. Pointer, F. Browand, M. Hammache, T. Y. Hsu, A.    Leonard, M. Rubel, P. Chatalain, R. Englar, J. Ross, D.    Satran, J. T. Heineck, S. Walker, D. Yaste, B. Storms, “DOE's Effort    to Reduce Truck Aerodynamic Drag-Joint Experiments and Computations    Lead to Smart Design” AIAA paper June 2004.-   14. Seifert, A., Stalnov, O., Sperber, D., Arwatz, A., Palei, V.,    David, S., Dayan, I., and Fono, I., “Large trucks drag reduction    using active flow control”, paper included in the proceedings of the    2nd heavy vehicle drag reduction conference, 2007. Eds. McCallen,    Browand. ISBN 978-3-540-85069-4. Pages 115-134.

Example 2 Efficiency Considerations

The present Example describes considerations directed to the enhancementof the energetic efficiency of unsteady fluidic actuators. Inparticular, the present Example describes considerations pertaining tothe selection of at least one of the oscillation frequency, the spatialdistance between the actuators in the array and the 3D distribution ofthe suction openings. A judicious selection of these parameters,according to some embodiments of the present invention can increase theenergetic efficiency, for example, enhanced (e.g., maximal) dragreduction can be obtained at relatively low (e.g., minimal) investedenergy.

The above parameters are associated with mechanisms that control andmanipulate the boundary layer of the body, its separation process, itsnoise and vibrations generation mechanism that can also be associatedwith the vortex shedding regime in the wake of the body.

The oscillation frequency is correlated directly to the frequency of theperiodic vortex shedding by the reduced frequency (the oscillations'Strouhal number). Certain oscillation frequencies, which in variousexemplary embodiments of the invention are uniform along the actuatorsin an array of actuators can be applied to trigger the vortex sheddingperiodic process from its natural uncontrolled state or even to suppressit altogether. This can significantly reduce energy losses whencontrolling the drag. These oscillation frequencies can be calculatedand then applied. Alternatively, an integer multiplication or divisionof the calculated frequency can be applied.

Judicious selection of the spatial distance between actuators cangenerate conditions for a specific spanwise wavelength which introducesstreamwise vortices into the boundary layer and near wake. This caneffect a change in the behavior of boundary layer and near wake in apredetermined manner. The suction pattern, place according to oneembodiment upstream of the pulsed blowing location can scale with thedistance between actuators to either enhance or attenuate the creationof the streamwise vortices and spanwise waves. It was found by thepresent inventors that these parameters can significantly affect atleast one of the separation delay capability of the AFC system, the dragreduction capability, and the energetic efficiency of the system. Insome embodiments of the present invention these parameters are selectedto suppress the vortex shedding, vibration and noise generation tendencyof the body.

Oscillations Frequency

A bluff body, in a free flow suffers from flow separation which isaccompanied by a periodic shedding of vortices from its separatedshear-layers. The frequency of the vortex shedding can be expressed bythe dimensionless Strouhal number (St) which correlates the vortexshedding frequency f with the free stream velocity v and the bodycharacteristic length L, via the relation St=f L/v.

For most common bluff bodies, the universal value of the Strouhal numberis between about 0.15 and 0.25. In some cases, the value of the Strouhalnumber is between about 0.05 and about 1. In many cases, correlating thefrequency with the wake width instead of the characteristic length makesthis value fixed at 0.2.

The pulsed or oscillatory blowing feature of the SaOB actuator blowsperiodically a jet tangentially, perpendicular or at another direction(e.g., upstream) with respect to the surface. Pulsed blowing openingscan be in the form of nearly 2D slots to circular holes. Theoscillations frequency of this jet can be determined by at least one ofthe following parameters: the supply pressure to the actuator and thedimensions of the feedback conduit length. To some extent, the presenceand features of a synchronization conduit or conduits also affects thefrequency. Therefore, by control of these parameters the oscillationsfrequency can be controlled.

In various exemplary embodiments of the invention the oscillationsfrequency for bluff body drag reduction is at least 2 or least 3 timeslarger the natural vortex shedding frequency. This is shown in FIG. 17which shows the relative drag reduction dependency on the reducedfrequency (left side ordinate). The right side ordinate in FIG. 17 showsthe momentum coefficient Cμ as a function of the reduced frequency F⁺,defined as the Strouhal number which is based on the excitationfrequency.

The length L_(FB) of the feedback conduit can be expressed in terms ofthe distance S from the control ports (e.g., from midway between thecontrol ports) to the splitter that separates the two exits from theoscillator (see, e.g., 56 in FIGS. 1A-1F). In some embodiments of thepresent invention the ratio L_(AB)/S is from about 2 to about 10.

In order to achieve a sufficiently high frequency (e.g., twice thenatural vortex shedding frequency) for low supply pressures, thefeedback conduit length is preferably selected relatively long. For highsupply pressures, the feedback conduit length is preferably selectedrelatively short. Thus, short conduits (e.g., conduits satisfying2≦L_(AB)<6) allow high frequencies at low supply pressure, and longconduits (e.g., conduits satisfying 6≦L_(AB)≦10) allow low frequenciesat high supply pressures.

Spatial Distance Between Adjacent Actuators

Due to the spatial 3D character of the SaOB actuator's blowing feature(according to one embodiment: periodic oscillations of the pulsed jetsin the span direction), at certain spatial distances between adjacentactuators (or its spacing along one of the cross flow dimensions of thecontrolled body), a spanwise structure in the form of spatial sine waveis formed. This wave is associated with the existence of streamwisevortices in the boundary layer and wake flow. These vortices have theability to manipulate or suppress the natural vortex shedding regimeand/or existing wake unsteadiness. Since the natural vortex shedding isan indicator of the wake stability, and it is associated with highvalues of drag coefficient, its reduction or suppression can effectivelyreduce the drag.

It was found by the present Inventors that relatively high dragreduction can be achieved for a spatial-spanwise (or cross-flow)directed wave length between about 0.5 and 4, more preferably between 1and 2, times the characteristic length. The characteristic length can bethe same length on which the Strouhal number is based, including,without limitation, the diameter of the cylinder (in embodiments inwhich the actuator has a shape of a cylinder or a section thereof), thewidth or height of the AFC system or the like.

In various exemplary embodiments of the invention the spatial distancebetween actuators is on the order of the characteristic length. It wasfound by the present Inventors that locating the actuators in thisspacing can generate the desired wavelength. In some embodiments of thepresent invention other values are selected for the spacing, dependingon the actuator geometry and the pressure supply to the actuator.

FIG. 18 shows the effect of the spatial waveform on the dragcoefficient. Comparison between identical conditions of momentumcoefficient and number of actuator is displayed. The elimination of thespatial wave was achieved by partly sealing of the suction holes, asdiscussed below. The vertical solid lines mark the position and width ofthe 3 actuators placed over the 270 mm span of the data shown above.

Partly Sealing the Row(s) of the Suction Holes

An additional tool for control of the spanwise structure of the nearwake flow is partly sealing of the suction holes. The segmented patternof the suction holes has the ability to enhance or suppress thewaveform. It was found that the magnitude of the spatial wave (ratherthan its length) is sensitive to the suction holes pattern. The presenceof spatial wave is associated with a significant drag reduction.Therefore, partly sealing the suction openings in conjunction with thelocation(s) of the pulsed blowing, which triggers strong wave atrelatively low energy investment is an essential complementarycapability to the spacing between adjacent actuators in the search formaximal energetic efficiency.

FIGS. 19A-19C illustrate several configurations for the opening of thesuction holes. In FIG. 19A, all suction holes open. This configurationis termed herein as the “All” configuration. In FIG. 19B only suctionholes between actuators are open. This configuration is termed herein asthe “Between” configuration. In FIG. 19C, only suction holes aboveactuators were open. This configuration is termed herein as the “Above”configuration.

FIG. 18 demonstrates the influence of the suction holes pattern on theability to generate a spatial wave. In this plot of two cases comparisona distinction is made between open suction holes between or aboveadjacent actuators where all the other parameters (number of actuators,momentum coefficient and orientation to the free-stream, e.g., flowcontrol location) are identical. The spatial wave is clearly shown andthe drag reduction is higher under its influence. When all holes areopen the effect is intermediate between the above two cases.

Synchronization of SaOB Actuators Array by a Single Set of Tube

Operation of actuators as a part of a synchronized array was proven bythe present Inventors to enhance the drag reduction and to deflect theflow better than a not-synchronized array. Synchronization of an arrayof SaOB actuators can, according to some embodiments of the presentinvention be using a plurality of parallel synchronization conduitsconnecting each side control ports. This configuration is shown in FIG.20A. In an alternative configuration, which is contemplated in someembodiments of the present invention the synchronization is byconnecting all the control ports in a serial connection using a singleconduit between control ports of adjacent actuators. This configurationis illustrated in FIG. 20B. In FIGS. 20A and 20B, control ports of thesame side are designated by the same name.

FIG. 20B illustrates a configuration in which the single conduitconnects ports of the same side (port A to port A and port B to port B).However, this need not necessarily be the case, since, for connectingopposite control ports of adjacent actuators (port A to port B).

Tests conducted by the present Inventors with both synchronizationmethods showed approximately perfect match of the frequency along theactuators in the array and enhancement of the drag reduction withsynchronized array. Synchronization by a single conduit (FIG. 20B) isadvantageous since the volume used for the synchronization is smallerfor single conduit than for a plurality conduit (half, when compared toa configuration with two synchronization conduits). Synchronization by asingle conduit (FIG. 20B) is also advantageous since it allowssynchronizing the actuators in a phase-lag by connecting oppositecontrol ports to each other (e.g., connecting each B port to the nearestA port).

Although the invention has been described in conjunction with specificembodiments thereof, it is evident that many alternatives, modificationsand variations will be apparent to those skilled in the art.Accordingly, it is intended to embrace all such alternatives,modifications and variations that fall within the spirit and broad scopeof the appended claims.

All publications, patents and patent applications mentioned in thisspecification are herein incorporated in their entirety by referenceinto the specification, to the same extent as if each individualpublication, patent or patent application was specifically andindividually indicated to be incorporated herein by reference. Inaddition, citation or identification of any reference in thisapplication shall not be construed as an admission that such referenceis available as prior art to the present invention. To the extent thatsection headings are used, they should not be construed as necessarilylimiting.

What is claimed is:
 1. An active separation control system, comprising afluidic oscillatory actuator having an ejector member and an oscillatormember both mounted on at least one flexible member, said fluidicoscillatory actuator being mountable on a rotatable door of a vehiclesuch that said flexible member assumes a different shape when said dooris closed than when said door is open.
 2. The system of claim 1,comprising at least two flexible members wherein said ejector member isattached at an inner part of one flexible member and said oscillatormember is attached at an inner part of another flexible member.
 3. Thesystem of claim 1, wherein each actuator has a joining channel, a pairof control ports at opposite sides of said joining channel, and afeedback conduit directly connecting said control ports of said pair toeach other.
 4. The system of claim 3, wherein said joining channel isflexible.
 5. The system of claim 3, wherein said feedback conduit isflexible.
 6. The system of claim 1, wherein there is a plurality offluidic oscillatory actuators, and the system comprises asynchronization conduit being mounted on said flexible member to connectat least two of said actuators such as to effect synchronization betweenoscillations in said at least two actuators.
 7. The system of claim 6,wherein said synchronization conduit is flexible.
 8. The system of claim6, wherein said synchronization conduit is effective to provide saidsynchronization devoid of any moving non-fluidic parts.
 9. The system ofclaim 6, wherein each oscillatory actuator comprises two opposingcontrol ports, and wherein each control port of each oscillatoryactuator is respectively connected to at least two control ports of atleast two another oscillatory actuators.
 10. The system of claim 6,wherein each oscillatory actuator comprises two opposing control ports,and wherein each control port of each oscillatory actuator is connectedto only one control port of another oscillatory actuator.
 11. The systemof claim 6, wherein at least one of said actuator is a suction andoscillatory blowing actuator (SaOB).
 12. The system of claim 6, whereinsaid synchronization conduit is selected to control phase lag among saidactuators.
 13. The system of claim 6, wherein said at least onesynchronization conduit is constituted to effect opposite oscillationsamong at least one pair of oscillatory actuators.
 14. The system ofclaim 6, wherein a frequency of said oscillatory actuators is X times anexpected vortex shedding frequency of fluid at the vicinity of thesystem, wherein X is from about 1.5 to about 3.5.
 15. The system ofclaim 14, wherein said X equals
 2. 16. The system of claim 6, wherein aseparation between adjacent fluidic oscillatory actuators is about X vSt/f, wherein v is an expected velocity of fluid at the vicinity of thesystem, f is an expected vortex shedding frequency of fluid at thevicinity of the system, St is an expected Strouhal number characterizingfluidic oscillations, and X is from about 0.5 to about
 4. 17. The systemof claim 6, wherein said vehicle is a truck trailer.
 18. The system ofclaim 6, wherein said vehicle is an aeronautical system.
 19. A vehicle,comprising the system of claim
 6. 20. A method of controlling air flowat a surface of a vehicle, the method comprising: mounting an activeseparation control system on a rotatable door of the vehicle, saidactive separation control system comprising a fluidic oscillatoryactuator having an ejector member and an oscillator member both mountedon at least one flexible member, such that said flexible member assumesa different shape when said door is closed than when said door is open.